Conventional construction of fuel cell and electrolyzer stacks, especially proton exchange membrane (PEM) stacks, require a large number of flat components (including bipolar plates, membrane and electrode assemblies, and, optionally, cooling plates) to be assembled between a pair of heavy metal endplates. The entire assembly is placed in compression through the use of a series of long threaded metal rods (tie rods) extending from one endplate of the assembly to the other endplate with nuts or other fasteners on either end.
FIG. 1 is a side view of an electrochemical stack 10 built using a conventional stack design. A cell stack 12 is disposed between two endplates 14. The cell stack and endplates are compressed by extending metal rods 16 from one endplate to the other endplate and fastening the ends of the rods, such as with bolts 18. The type of design depicted in FIG. 1 is often referred to as a "filter press" design.
While conventional "filter press" designs may be straight-forward, and effective, they are also bulky and heavy. In conventional "filter press" designs the entire load is applied by the bolts along the edges of the stack. In order to compress the stack as evenly as possible over the cross-sectional area of the stack without bending the end plates, the endplates are made very thick. While increasing the thickness of the end plates may help make them rigid, increasing endplate thickness results in an increased total weight of the electrolyzer stack. Another contributing factor to the increased weight in the "filter press" design is the necessity to place the tie rods around the perimeter of the active portion of the stack, thereby requiring endplates that are even larger in area than the stack.
The size of the end plates can be marginally reduced by placing the tie rods inside of gas passages, and therefore, inside the area of the bipolar plates. While this type of design allows reduction in electrolyzer stack-weight, the reduction is limited by the continuing need for heavy rods and rigid endplates in this type of design.
FIG. 2 is a cross sectional view of a "filter press" type stack 20, similar to the design of FIG. 1, with a "spider" 22 added to improve the distribution of the closing force. The tie rods 24 pass freely through the endplates 26 and around or through the stack 28. In this design, tightening the nuts 25 on the ends of the rods 24 pulls down on the lever arms 27 of the spider 22 and transfers the force to the center of the stack. The design of FIG. 2 allows for leveling and distributing the load away from the edges by including a floating load distribution feature in the endplate design.
There are two separate sets of tie rods in the "filter press" design of FIG. 2. One set of rods (not shown, but similar to those in FIG. 1) secure the endplates against the stack, with all of the force applied at the corners. The other set of rods 24, pass freely through the endplates and the stack. When the nuts on the tie rods are tightened, the totality of the force is applied to the endplates at locations imposed by the design (e.g. the center of the endplate as shown in FIG. 2). While this approach does improve the distribution of the closing force over the area of the stack, and may assist in keeping the center of the stack in compression, it also leads to an increase in unproductive stack volume, and in the weight of the endplate assemblies. The weight reductions offered by improvements such as those described in relation to the "filter press" design of FIG. 2 are significantly limited.
Therefore, there is a need for an assembling device or system that compresses stacks without adding much weight and volume. Further, it would be desirable if the assembling devices provided more flexibility and control in adjusting the degree and distribution of compression.